Biomining makes use of microorganisms in processes aimed at extracting metals from sulfide- and/or iron-containing ores and other mineral concentrates. For example, biomining can be used to recover gold from certain ores. That is, although gold is inert to microbial action, microbes can be used in processes to recover gold from certain ores because the microbes oxidize the ore thereby opening the structure of the ore, which allows gold-solubizing chemicals, such as cyanide, to penetrate the ore.
Biomining is particularly helpful when attempting to retrieve minerals like gold from “refractory” ores. With such refractory ores, conventional fine grinding and cyanidation are not as efficient at retrieving the desired minerals from the ore as biomining. Further, biomining with biooxidation, compared to other oxidation mining techniques, such as roasting and pressure oxidation, is relatively simplistic, requires only mild operating conditions, has low capital costs, requires relatively low amounts of energy, and is friendly toward the environment.
As discussed in Fernando Acevedo's article on “The use of reactors in biomining processes,” EJB ELECTRONIC JOURNAL OF BIOTECHNOLOGY, Vol. 3, No. 3, Dec. 15, 2000, available at http://www.ejb.org/content/vol3/issue3/full/4, biooxidation of refractory gold ores with the use of heap operations or tank reactors is well known in the field of biomining. However, each method of biomining has its negatives. For example, though heap operation is simple and adequate to handle large volumes of ores, the productivity and mineral yields are limited because of the inherent difficulty of adequately controlling the process conditions of the heaps. Tank reactors, on the other hand, can handle moderate volumes of ore to be processed, allow for close monitoring and control of process conditions, and render significantly better performance yields than the heap operations. However, the costs for biomining with tank reactors are significantly higher than the costs for biomining with heap operations. The additional cost of the tank reactor methods may be justified only when the ore input to the reactors have a high value of mineral concentrate.
In any regard, during the biooxidation process of gold-containing ore, bacteria, such as those belonging to the Thiobacillus or Leptospirillum genera, partially oxidize sulfide coatings covering the gold micro particles contained within the ores. These microorganisms are capable of dissolving sulphides, such as pyrite and arsenopyrite, so as to liberate the otherwise-encapsulated gold particles within the ore. Following subsequent processing, such as through cyanidation, gold yields from the ore have been known to reach levels of approximately 85-95%, as opposed to yields of 15-30% when biooxidation is not utilized.
As Fernando Acevedo explains in his article,                [s]everal mass transfer operations occur in a biomining operation. Nutrients have to reach the attached and suspended cells, metabolic products have to migrate from the cells to the liquid and solubilized species must be transported from the surface of the mineral particles to the liquid. In addition, two other important transport processes are to be considered: the supply of oxygen and carbon dioxide from the air to the cells. Carbon dioxide is demanded by the cell population as carbon source, while oxygen is needed as the final electron acceptor of the overall oxidation process. In reactors these gases are usually supplied by bubbling air into the liquid. In order to be used by the cells, oxygen and carbon dioxide must dissolve in the liquid, a mass transfer operation that presents a high resistance and can become limiting for the overall process rate.Fernando Acevedo, The Use of Reactors in Biomining Processes, Vol. 3, No. 3, EJB ELECTRONIC JOURNAL OF BIOTECHNOLOGY, Dec. 15, 2000, available at http://www.ejb.org/content/vol3/issue3/full/4. Accordingly, agitation of the ore to be bio-mined via a bacteria-containing slurry is beneficial to increase transfer rates, including the rates of transfer of oxygen and carbon dioxide as well as heat transfer. Agitation also discourages stagnation of the materials in the bio-reactors and reduces occurrences of unwanted zones of reactor contents with insufficient nutrients, inadequate temperatures, or inadequate pH levels. Ideally, bio-reactors allow for optimal agitation and eliminate stagnant zones within the reactors.        
Thus, what is needed is a bio-reactor having agitation features that can economically accommodate biooxidation of significant volumes of mineral-containing ores while producing high levels of mineral yields.